Vibrating wire sensors, also known as acoustic strain gages, have become the most widely used instrument for construction monitoring. For example, vibrating wire sensors have been widely used for measurements in such civil structures as bridges, dams, and buildings. They have been used to monitor stress, strain, deflection, pressure, displacement, fluid level, angular motion, and temperature. Even as advancing technology has produced many types of sensors based on different technologies, the construction and civil engineering industries still view vibrating wire sensors as the best type of sensor for long-term reliability. They are the most trusted and familiar tool in the civil engineering field. They have also been used for measurements on other substrates, such as aircraft and other vehicles, machinery, and pipelines.
Vibrating wire sensors have generally been considered to be quite accurate, able to resolve as little as 0.1 microstrain. They are also robust, packaged to resist corrosion and withstand extreme environmental conditions.
The output of a vibrating wire sensor is an AC electrical signal with a frequency corresponding to the frequency of the vibrating wire. Although amplitude of this signal may deteriorate as this electrical signal is transmitted over a long wire the frequency is maintained, and so the measurement has been especially suitable for wired transmission over long distances.
Vibrating wire sensors include end anchors holding a wire in tension, as described in U.S. Pat. No. 4,074,565, to Harris et al., “Vibratory-Wire Strain Gage,” incorporated herein by reference. Typical prior art vibrating wire sensor 22 is also illustrated in FIG. 1. End anchors 24 are attached to structure 26 or other object being measured. These end anchors separate from each other or approach each other as the structure or object experiences forces and moves in response to those forces. The change in separation of the end anchors imparts a change in the natural or resonant frequency of wire 30, going to a higher pitch as anchors 24 are separated and to a lower pitch as they are brought closer together.
One way the resonant frequency of wire 30 has been measured has been to pluck the wire and then to measure the natural frequency of the vibration of the wire as it vibrates freely. Wire 30 has been plucked by fabricating the wire of a magnetically permeable material, such as steel, and providing current in coil 32 mounted adjacent wire 30, as also shown in FIG. 1.
One way of plucking the wire has been to provide sufficient current through the wire to provide a strong enough magnetic field to displace the center of the wire from its resting position. When the current is turned off the wire is released and vibrates.
Another way has been to provide an AC current in the coil that varies with time over a range of frequencies. This scheme recognizes that the wire would readily vibrate, even with a very low energy plucking signal, if the signal is provided at a frequency equal to or sufficiently close to the wire's resonant frequency. However, because the ‘Q’ of the resonance of the wire in the vibrating wire gage has been extremely high to provide the greatest measurement resolution, the wire in the vibrating wire strain gage is very sensitive to the frequency of the plucking signal. For example, if the excitation frequency is exactly the same as the natural vibration frequency of the vibrating wire, or is within about 1 Hz of the resonance frequency of the vibrating wire, very little power has been needed to excite the wire to vibrate at its resonance frequency. However, if the excitation frequency is more than about 1 Hz away from the resonant frequency of the vibrating wire, a large amount of power into the excitation coil has been required to ‘ring’ the wire at its natural resonance frequency to a level that can be detected. Thus, when the frequency of the AC current in the coil has been too far from the natural frequency of vibration of the wire, the wire has not vibrated. In this scheme the frequency of the AC current in the coil has been varied until a frequency that is about equal to the natural frequency of frequency of vibration of the wire, or a harmonic, has been reached, which gets the wire vibrating.
Thus, vibrating wire 30 has been plucked by techniques that require a considerable amount of power to be switched into the coil either with a single large current or a succession of AC signals at different frequencies until the right frequency has been reached.
After the plucking signal stops, wire 30 vibrates freely at its resonant frequency for quite some time. The natural frequency of vibration has been detected by a pickup mounted adjacent to the wire. The pickup usually includes permanent magnet 33 located in the center of coil 32. The coil used for pickup can be the same coil as used to pluck the vibrating wire. Alternatively the pickup can be a separate coil from the coil used to pluck the vibrating wire. As steel wire 30 vibrates in relation to the permanent magnet the magnetic circuit coupling between the wire and the permanent magnet changes, inducing an AC current in coil 32 that oscillates with a frequency equal to the frequency of vibration of wire 30. The pickup has been connected to counter 34 through cable 36, and counter 34 determines the frequency of vibration of wire 30 from the frequency of the electrical signal it receives.
Remote reader 38 including hefty power supply 40 has previously been needed to supply the high current level needed for either of the two types of plucking signal. This plucking signal has been transmitted from remote reader 38 over a pair of wires located in cable 36 extending from remote reader 38 to coil 32. Similarly, the electrical signal induced in coil 32 as a result of the vibration of the wire has been conducted back to frequency counter 34 in reader 38 over the same pair of wires in cable 36. In some cases the cable connecting the vibrating wire gage and reader has been many hundreds of feet long. The remote reader has included electronic components for signal conditioning, including amplification, processing, display, recording and counting.
Plucking with either plucking techniques has required relatively high power and energy consumption. Most manufacturers of these available signal conditioners use such large signal pulse excitation, consuming more than about 100 mJ of energy for each reading.
With the need to be able to supply a large amount of energy for plucking the vibrating wire a wired connection to a source of such a large signal has been needed, and there has been no practical way to provide a wireless vibrating wire sensor.
Another important characteristic of vibrating wire sensors has been their sensitivity to temperature. For example, with a steel wire, which has an expansion coefficient of about 11 ppm, a change of 1 degree Celsius produces eleven times as much change in wire resonant frequency as a change of 1 microstrain. Therefore, it has been important to compensate for changes in temperature to provide data that truly indicates the change in strain of the structure. For this reason vibrating wire sensors have been manufactured with a thermistor built into the excitation pickup coil assembly so that temperature can be measured along with frequency and so the frequency measurement can be adjusted for temperature. The thermistor has required its own wire for connection to the reader to accurately provide the temperature reading.
Although vibrating wire sensors have been subject to improvement over the past 60 years since they were first introduced, current technology vibrating wire gages have been large and consume considerable power. The large size and the high power consumption of vibrating wire signal conditioners has prevented the emergence of satisfactory wireless units. The need for the wiring to provide the power from the reader needed for plucking the wire, to transmit the frequency data back to the reader, and to transmit temperature data back to the reader has added considerably to the cost of using the gages, limited the number of gages that could be provided to monitor a structure, limited the types of structures that can be monitored, limited the frequency and duration of monitoring, and limited the ability to monitor during actual operation.
It is worth noting that one of the biggest issues arising concerning vibrating wire sensors on a construction site relates to cabling for the sensors. In many instances vibrating wire sensors are located in areas that are difficult or dangerous to access, hence long cables frequently connect the sensors to remote readers. Cable routing has to be planned carefully to ensure that cables can be protected. The cost of the cable can often add 50 to 100% of the cost of the vibrating wire sensor. The cost of designing the routing, installing the cable, and providing and installing suitable conduit or other protective measures can add an additional 100 to 400% of the cost of the vibrating wire sensor.
Thus a better scheme is needed to reduce size, reduce power, reduce or eliminate the wiring, reduce complexity, and reduce cost, and this scheme is provided by this application.